Dicyandiamide (DICY) has been used for a long period of time as a curing agent of epoxy resins for printed wiring boards. However, a material superior in a long term insulation reliability (CAF resistance) and a material with a high decomposition temperature required for lead-free soldering have been demanded as a requirement along with higher density wiring in printed wiring boards in recent years, so that phenol type curing systems superior in these properties have been used as the curing agent.
However, phenol type curing agents have problems such that they cannot impregnate well into a glass base substrate and prepregs prepared therefrom are poor in appearance.
In regards to improvement of appearance in prepregs, it is described in Japan Patent Kokai H07-48586 and Japan Patent Kokai H07-68380 that impregnation into the prepreg is improved by reacting tetrabromobisphenol A with both a bisphenol A type epoxy resin and a novolac type epoxy resin in preparation of prepregs; and it is described in Japan Patent 3395845 that appearance of prepregs is improved by using a bisphenol novolac resin with a softening temperature of 60° C. to 90° C.